The relative stabilities of the R2XO, trans-RXOR, and cis-RXOR isomers (X = C, Si, Ge, Sn, Pb; R = H, CH3) were investigated by ab initio and DFT pseudopotential calculations. Geometry optimizations and frequency computations were performed at the MP2 and Becke3LYP levels with DZ+P basis sets. MP4SDTQ/(DZ+P)//MP2/DZ+P and CCSD(T)/(DZ+P)//MP2/DZ+P single point calculations gave high level ab initio data. The CCSD(T)/(DZ+P)//MP2/(DZ+P) and the Becke3LYP/(DZ+P) relative energies both agree and show that the C 2v H2XO isomers (with the exception of formaldehyde) are the least stable and that the cis- and trans-HXOH Cs isomers are very close in energy. Acetone and dimethylsilanone are the most stable isomers in the Me2XO series. However, with the notable exception of HPbOH, the H2XO compounds are not stable thermodynamically toward cyclotrimerization or to hydration. The NPA/NLMO charges, bond orders, and orbital hybridizations support the polar description -O:::X+R2 of all metal “double bond” species. The -O:::X+ symbol implies the polarization of both π and σ bonds, which are stabilized electrostatically by methyl substitution. Only weak π interactions occur in the carbene-like RXOR metal isomers, and the σ bonds are mainly formed by p orbitals. In contrast, the X s orbitals participate strongly in the σX-O bonds of the -O:::X+ systems; the oxygen hybridization is far from sp2.

Very strong cation stabilization results from x conjugation, for example, from the interaction between empty carbon 2p orbitals and the lone pairs on nitrogen-or oxygen-based substituents.['-31How do the heavier group 15-17 substituent lone pair donors compare with their lighter analogs? It has been recognized for some time that TC overlap arising from elements of different rows is not reduced due to differences in the p-orbital Conclusions from previous work are contradictory. While the amino group is the most stabilizing x-donor substituent, it is not clear if sulfur or oxygen is the better n do no^-.[^-''^ The 0,' constants, PhO = -0.62, PhS = -0.54, and PhSe = -0.47 are not very different but show a progression down the group." 2, 13] The trend in the calculated hydride affinities of +CH2XH (X = 0, S, Se. Te) is similar, with a sharp difference between oxygen and sulfur." ' cl-halomethyl cation stabilization energies that increase down the group: F < C1< Br

No abstract

The "cyanide" salts of the group 2 (alkaline earth) metals exhibit remarkable structural variations: CN(-) binds to the metals via the carbon, via the nitrogen, and via bridged arrangements. The most stable geometries of the beryllium and the magnesium salts are linear (CNBeNC and NCMgCN, respectively), but CaC(2)N(2), SrC(2)N(2), and BaC(2)N(2) prefer twisted, bridged structures. However, several stationary points of the bridged complexes are close in energy, and considerable fluxionality is to be expected. These theoretical predictions (MP4SDTQ/6-311+G(2d)//MP2(fu)/6-31+G, Ca, Sr, Ba: 5s5p3d1f//5s5p3d basis sets and 10 valence electron pseudopotentials) invite experimental verification.

In erster Linie von der Gruppe und weniger von der Periode hängt der stabilisierende Einfluß von Elektronenpaardonor‐Substituenten der ersten fünf Reihen des Periodensystems auf Carbokationen ab. Dies zeigen ab‐initio‐Rechnungen auf hohem Niveau (QCISD(T)/DZ+ +PP//MP2(fc)/DZ + P). Wird die EH2‐Planarisierung (E = Element der Gruppe 15) berücksichtigt, sind die Stabilisierungsenergien durch Substituenten der Gruppe 15 nahezu konstant (ca. 95 kcal mol−1 von N bis Sb relativ zu CH+3); sie übertreffen die Methylstabilisierungsenergien der Chalkogene (60–66 kcal mol−1, O < S ≈ Se < Te) und der Halogene (18–38 kcal mol−1, F < Cl ≈ Br < I).